This invention relates generally to placing cementitious material such as concrete in foundations. Foundations for buildings, bridges, and other structures can be constructed in several different ways. Two categories are shallow foundations and deep foundations. Examples of shallow foundations include mats and spread footings. Examples of deep foundations include driven piling made of either concrete or steel and drilled shaft methods. The present invention relates to the drilled shaft methods.
A piling may be a pre-cast or pre-fabricated member made of either steel or concrete that is driven with a hammer into the ground until it attains a certain refusal rate which indicates that the piling can support a certain load. Some soils are so hard or consolidated, however, that they may not readily accept a driven pile or the cost is greater than the drilled shaft method. Also, certain engineers may prefer one type of foundation system over others and surrounding conditions may make one type of foundation system preferable or necessary.
In the drilled shaft method, the shafts are drilled with foundation drilling equipment, which may comprise an attachment on a crane or a truck-mounted type of rotary. The rotary turns a Kelly-bar or drill-stem, which is attached to an auger or drill bucket or some other drilling bit, which excavates the hole. As the hole is drilled, the cuttings are brought to the surface and disposed on the ground. This process is continued until the shaft is excavated to the required depth.
Once the hole is drilled to the desired depth then a reinforcing cage is normally placed in the hole. The length of the steel may or may not extend to the full depth of the hole. Reinforcing cages are typically of a rebar type of construction, with spiral or circular hoops. Hoops and rebars are tied into a fabricated cage. The cage is then lifted up by a crane and placed in the hole. It either rests on the bottom or is suspended at the top of the hole by some method.
The drilled shaft method can be practiced several different ways. In one way called the dry method, an open hole is excavated and the sides of the shaft are self-supporting. The foundation material is then merely placed into the hole. Another method is the cased method, where a steel casing is used throughout the area of an unstable soil strata. The excavation may then proceed through and below the casing in most cases.
A third way of practicing the drilled shaft method is the wet hole or slurry-displacement method where the walls of the shaft are kept open and prevented from collapsing by use of a drilling fluid such as a bentonite slurry, atapulgate, a polymer solution, or water. These fluids exert a hydrostatic pressure to keep the hole or excavation from caving. Sometimes, water is present in the excavation not because of a need for caving prevention but because certain formation stratus communicate natural water into the excavation. In the slurry-displacement method, which is one method to which the present invention relates, ready-mix concrete is placed in the fluid filled excavation from the bottom up. A steel pipe called a tremie pipe is used to place the concrete into the hole.
A tremie pipe is usually a small-diameter pipe that extends from the top of the shaft or excavation to the bottom and through which concrete is poured from the top. A pig, or foam-rubber plug is often put in ahead of the concrete to push the water or other drilling fluid out of the pipe ahead of the concrete. The bottom of the tremie can also be temporarily plugged water tight and filled with concrete. The concrete has a specific gravity greater than the drilling fluid. As the concrete is poured, the fluid is displaced to the top. It either runs out of the excavation on the ground or pumped to a holding tank or pit for later reprocessing or disposal. To pour these shafts, a crane or other lifting apparatus is often used to hold the tremie pipe in the hole.
A tremie pipe may be made of sections about ten (10) feet long and of small-diameter casing in the order of ten (10) inches, each section having threaded ends. These threaded ends are screwed together until a tremie pipe of sufficient length to reach the bottom of the hole is produced. A hopper may be placed at the top of the tremie pipe for receiving concrete which then flows down the pipe. As the excavation is filled, the tremie is periodically raised and a joint is removed. The bottom of the tremie remains submerged in the fluid concrete.
In the case of a solid piece of tremie pipe, without any joints at 10-foot intervals such as earlier described, a concrete bucket or a concrete pump may be used to help elevate the concrete into the hopper once the tremie is raised up. In order to prevent the concrete, which becomes progressively more resistant to flow, from lifting the steel reinforcing cage up, the tremie pipe is raised as the concrete is placed. Thus, it may be necessary to use two cranes: one to the hold and raise the tremie pipe and another crane with a concrete bucket to deliver the concrete. A concrete pump boom truck may also be used to support the tremie pipe when the size and length of the tremie pipe permits.